Corrosion resistant ferrous metal articles and method of preparing the same



United States Patent. ce

3,526,486 CORROSION RESISTANT FERROUS METAL ARTICLES AND METHOD OFPREPARING THE SAME Edwin J. Smith, Winterville, Ohio, Louis C. Beale,In, Coraopolis, Pa., and Lowell W. Austin, Weirton, W. Va., assignors toNational Steel Corporation, a corporation of Delaware No Drawing. FiledFeb. 21, 1967, Ser. No. 617,513 Int. Cl. C23b 5/06; C23f 7/26, 17/ 00'US. Cl. 29-1835 10 Claims ABSTRACT OF THE DISCLOSURE Corrosion resistantferrous metal articles are prepared by electroplating thereon a metallicchromium coating having a thickness of 0.1-0.5 microinch, and thenelectrochemically treating the article as a cathode in an aqueouselectrolyte containing a water soluble hexavalent chromium compound todeposit a chromium oxide containing film. The resultant surface isreceptive to organic coatings and the invention is especially useful inthe manufacture of thin gauge tin free strip or sheet for use ascontainer stock.

This invention broadly relates to corrosion resistant ferrous metalarticles and a method for preparing the same. In some of the morespecific variants, the invention is concerned with thin gauge tin freesteel sheet material which is useful as container stock and a method forits preparation.

Tinplate is widely used at the present time for the manufacture ofcontainers of the type used in the storage and preservation of foods andbeverages. However, due to the high cost of tin and the lack of adependable source of tin in this country, it is desirable to provide lowcost corrosion resistant tin free steel strip or sheet for use ascontainer stock.

A number of efforts have been made heretofore to pro vide satisfactorytin free strip or sheet for use as container stock. Inasmuch as a cleansteel surface is subject to rapid corrosion, some type of an initialprotective coating must be applied at the time of manufacture. Theinitially protected steel surface must be receptive to the usual organiccoatings that are applied to tinplate such as paints, varnishes,lacquers and enamels to thereby provide relatively permanent protectionagainst corrosion. The steel surface should also be receptive to theorganic adhesives which may be used in joining the side seams ofcontainers prepared therefrom. Thus, satisfactory tin free steel sheetmaterial for use as container stock should have a surface which has acombination of desirable characteristics, including the following:

1) The steel surface must have an initial protective film or coatingthereon which retards corrosion, at least from the time of manufacturein the form of thin gauge strip or sheet until it is organically coatedand is ready for use in the preparation of containers.

(2) The initially protected steel surface should have a pleasing brightsilver to light grey appearance, as distinguished from a dark ordiscolored appearance, as it is often desirable to apply clear organicprotective coatings thereto.

(3) The initially protected steel surface must be receptive to organicprotective coatings of the usual types applied to tinplate and'must forma good base therefor. For example, the organic coatings must adheretightly to the steel surface and provide good underfilm corrosionresistance.

3,526,486 Patented Sept. 1, 1970 (4) The organically coated steelsurface should pass the peel and side seam adhesion tests, as the sideseams of tin free containers are often formed by cementing the sideedges together with a resinous organic cement. If the overlapped edgeportions cannot be tightly adhered together with the organic cement, theside seams separate and the container fails.

This is a highly competitive field and in addition to the aboverequirements, the initially coated steel strip or sheet must complete inprice with other low cost materials.

In fact, the competition is so keen that the method used in initiallytreating the steel surface must not add appreciably to the overall costof manufacture of blackplate. A wide variety of methods have beenproposed heretofore for protecting the steel surface initially. However,the treatments and/or coatings proposed heretofore have had somedisadvantage or deficiency which prevented them from being satisfactoryin all respects.

One of the treatments that has been proposed heretofore involveselectrodepositing a substantially nonporous layer of metallic chromiumon ferrous metal sheet material for the purpose of producing a substratesurface which is sufficiently corrosion resistant, and also receptive toorganic finishes. However, relatively thick chromium coatings wererequired of the order of 2 microinches or thicker to obtain sufiicientcorrosion protection before the organic coating was applied. This has atleast two disadvantages, one being the high cost of applying thechromium plating. As is well known, chromium plating is very inefficientand only a small fraction of the electric current is effective inplating out metallic chromium. Additionally, the relatively thickchromium coatings often are not sufficiently ductile and cracks appeartherein when the substrate is subjected to drastic forming, such as inthe manufacture of containers therefrom.

It has been discovered that very thin coatings of metallic chromium maybe electrodeposited on ferrous metal substrates, and although suchinitially deposited chromium coatings are porous and are unsatisfactorywithout further treatment, this problem may be readily overcome by aninexpensive electrochemical treatment of the coated substrate. Theresultant substrate is an excellent base for organic coatings, and it isfully equivalent in this respect and in initial corrosion resistance tosubstrates having the much heavier chromium coatings of the prior art.

It has been further discovered that the very thin layers of. metallicchromium which are deposited in accordance with the invention areductile and formable, and are not subject to cracking during a formingoperation on the substrate such as when side seams are formed in themanufacture of containers. Thus, the present invention provides for thefirst time an entirely satisfactory inexpensive method for the treatmentof ferrous metal strip or sheet to provide high quality container stock.

It is an object of the present invention to provide improved corrosionresistant ferrous metal articles and a novel method of preparing thesame.

It is a further object to provide a novel method of improving thecorrosion resistance of ferrous metal substrates by electroplating athin layer of chromium thereon, and then electrochemically treating thecoated substrate to produce a corrosion resistant surface which isreceptive to organic coatings.

It is still a further object to provide improved corrosion resistantcontainer stock which is useful in the manufacture of containers and anovel method of preparing the same including applying an organic coatingsuch as paint, varnish, lacquer or enamel to the initially treatedferrous metal substrates of the invention.

Still other objects and advantages of the invention will be apparent tothose skilled in the art upon reference to the following detaileddescription and the example.

In accordance with the present invention, the corrosion resistance offerrous metal articles is improved by electroplating at least a portionof the surface area thereof with a coating of metallic chromium having athickness of 0.1-0.5 microinch, and then electrochemically treating thechromium coated surface as a cathode in an aqueous electrolytecontaining a water soluble hexavalent chromium compound. Thereafter, thetreated ferrous metal surface may be coated with an organic finish toprovide relatively permanent protection against corrosion. There arecertain preferred variants of the invention which produce superiorresults, as will be illustrated and described in gerater detailhereinafter.

Any suitable prior art chorimum plating bath may be used forelectrodepositing the metallic chromium layer. As is well known,chromium plating baths usually contain chromic acid or other watersoluble chromium compound and a catalyst such as sulfate ion, fluorideion, silicofluoride ion, or a combination thereof. The catalyst may beadded as a free acid providing the desired ion, such as sulfuric acid,or as a water soluble salt such as the alkali metal compounds. The molarratio of chromic acid to the catalyst is usually about 100:1. Thechromic acid content of the bath may range from about 100 to 400 gramsper liter, and is preferably about 150- 300 grams per liter. The currentdensity may be the same as used in prior art chromium plating, such asabout 300- 2,000 amperes per square foot, and preferably about 500-1,000amperes per square foot. The bath temperature also may be in accordancewith the prior art, such as about 90-150 F., and preferably about110-130 F. Insoluble anodes are used, and preferably a composite steelanode prepared by applying a lead coating to the side facing the ferrousmetal substrate to be plated, and polyvinyl chloride or other inertinsulating substances to the opposite side to prevent or reduce straycurrents. The substrate surface is scrubbed free of oil, dirt and othersurface contaminants by a prior art process prior to electroplating withthe metallic chromium.

While the above plating baths and conditions are satitsfactory, it isunderstood that still other prior art chromium plating baths may beused. Examples of other plating baths and conditions are disclosed inUS. Pat. Nos. 1,942,469; 2,177,392 and 2,415,724, and in the text ModernElectroplating, edited by Frederick A. Lowenheim, 2nd ed., John Wiley &Sons, Inc., New York, N.Y., 1963. Chapter 5, pages 80-140 and thereferences cited on pages 128-140 are especially pertinent as thisportion of the text is directed to chromium plating. The teachings ofthis text, and the references cited therein, are incorporated herein byreference.

Regardless of the specific prior art chromium plating bath andconditions which are employed, it is understood that the coating ofmetallic chromium that is electrodeposited has a thickness of 0.1-0.5microinch. Unsatisisfactory results are achieved due to insufiicientmetallic chromium at coating thicknesses less than 0.1 microinch, whilecoatings having thicknesses above 0.5 microinch tend to crack duringforming into containers and the like, thereby decreasing the desirableproperties of the substrate rather than improving them. Additionally,chromium coatings above 0.5 microinch are expensive and add appreciablyto the cost of manufacturing tin free steel for container stock due tothe inefficiency of the chromium plating process. Best results areusually achieved by maintaining the metallic chromium coating at athickness of 0.1-0.3 microinch. A metallic chromium coating having athickness of 0.1 microinch is satisfactory for container stock to beused in the preparation of containers for the storage and preservationof less corrosive carbonated beverages and foodstuffs, whereas metallicchromium coatings of 0.3 microinch are pre- 4 ferred when storing andpreserving corrosive foodstuffs and carbonated beverages.

Metallic chromium coatings of 0.1-0.5 microinch in thickness have aporosity whereby the ferrous metal surface is subject to corrosion atthe pore sites. Thus such metallic chromium coatings do not providesatisfactory initial protection against corrosion of the ferrous metalsubstrate. As a result, heretofore these extremely thin metallicchromium coatings have not been considered to be useful in themanufacture of high quality container stock.

It has been discovered unexpectedly that metallic chromium coatingshaving a thickness of 0.1-0.5 microinch may be electrochemically treatedto render them satisfactory for container stock. The bath that is usedfor the electrochemical treatment may contain a water soluble hexavalentchromium compound such as chromic acid, alkali metal dichromateincluding sodium or potassium dichromate, etc. Chromic acid is oftenpreferred. Satisfactory baths may contain about 50-100, and preferablyabout 80, grams per liter of chromic acid, or about 2.5-3.5, andpreferably about 3 ounces per gallon of sodium or potassium dichromate.

The electrochemical treatment baths may be operated at a temperature ofabout -150 F., and preferably at about -130 F. The substrate may betreated cathodically under current conditions to provide about 25-200amperes per square foot, and preferably about 50-100 amperes per squarefoot, over a period of time to result in treatment with about 25-500coulombs per square foot, and preferably about 75-125 coulombs persquare foot. The optimum treatment is often at the rate of 50 amperesper square foot for 2 seconds to thereby provide 100 coulombs per squarefoot, using a bath temperature of about 100 F. The electrochemicaltreatment bath should not contain more than 20 parts per million byweight of chloride ion in the absence of additives to prevent stainingof the electrochemically treated surface. Also, higher levels ofchloride ion tend to catalyze chromium oxide formation in the film,which causes inferior peel test values and poor underfilm corrosion testresults in some instances.

As a general rule, about 0.6-5.0 milligrams per square foot of chromiumshould be present in the film that is deposited in the electrochemicaltreatment. The cathodic electrochemical treatment results in thedeposition of a chromium oxide-containing film which also containsmetallic chromium. The chromium content of the chromium oxide seems tobe largely in the plus three valence state, and may include hydrated CrO in most instances. Preferably, the film should contain about 1-3milligrams per square foot of total chromium in the metallic chromiumand chromium oxide contents of the film and for best results about 1-2milligrams per square foot.

Surprisingly, the electrochemical treatment markedly reduces theporosity of the initial metallic chromium coating. It is thought thatfor some as yet unexplained reason the metallic chromium content of theelectrochemically deposited film tends to seek out the pores in theinitially deposited metallic chromium film, and thereby renders it lessporous. In addition to this, the chromium oxide content of the film andthe metallic chromium dispersed therein tends to cover and furtherprotect the initial metallic chromium coating where it is continuous tothereby markedly increase the corrosion resistance. At the same time, asurface is produced which is highly receptive to the prior art organiccoatings. The organically coated electrochemically treated surface alsois an excellent base for the side seam adhesives which are sometimesused in the manufacture of containers, and the peel test and the sideseam adhesion test results are far above minimum specifications.

Organic coatings of the types usually applied in the prior art to tinfree steel or tinplate to further improve the corrosion resistance, andto provide relatively permanent protection for the initially protectedferrous metal base, may be applied over the electrochemically treatedsurface to produce corrosion resistant container stock. While theinvention is not limited thereto, examples of suitable organic coatingsinclude phenolic, modified phenolic, epoxy, modified epoxy, vinyl resin,Teflon and drying oil based paints, varnishes, lacquers and enamels. Theselection of a specific organic coating to be employed in a giveninstance is within the skill of the art.

The present invention is especially useful in the treatment ofblackplate strip or sheet, herein referred to as sheet material orferrous metal sheet material, of, the usual gauges used in tin freesteel or tinplate manufacture. Backplate having a weight of 55-90 poundsper base box is often used for container stock to be used in thepreparation of containers for the preservation and storage of foods andcarbonated beverages. However, the treatment of the invention may beapplied to any ferrous metal surface.

The ferrous metal article to be treated need not be given a specialpretreatment prior to deposition of the metallic chromium coating. Thesurface, however, should be free of rolling oil, rust, dirt, scale andother foreign matter, and it is preferred that the article be cleaned bythe usual prior art steps used in the manufacture of chromium plate. Ininstances where the substrate is blackplate strip, the strip may bepassed continuously through an electroplating line at high speed andelectrolytically cleaned in a caustic cleaner, such as anodic andcathodic treatment in an Orthosil solution, rinsed with water,thereafter pickled in dilute sulfuric acid or hydrochloric acid,scrubbed and rinsed with water, and then plated with metallic chromium.After plating with the metallic chromium, the strip may be continuouslyrinsed with water, and then immersed in the electrochemical treatmentbath where it receives the chromium oxide-containing film. If desired,the strip entering the plating bath and/or the chemical treating bathmay be prewetted with electrolyte by means of sprays or the likearranged above the baths, and/or counter current rinsing may be used torecover dragout electrolyte for recycle, as is disclosed in U.S. Pats.Nos. 2,648,625 and 2,825,681 and British Pat. No. 546,269. Thereafter,the strip may be continuously rinsed with hot or cold water, dried,oiled lightly with dioctylsebacate or other suitable lubricant such asis used in tinplate manufacture, and coiled. The strip may be coatedwith an organic finish continuously following prior art procedures, andthe resultant strip cut into suitable lengths for the manufacture ofcontainers therefrom. Alternatively, the organic coating may be appliedto the dried strip following the electrochemical treatment and theoiling step may be eliminated. Also, the electrochemically treated stripmay be cut into sheets of a desired size, then organically coated, andcontainers made therefrom, or containers may be prepared from theelectrochemically treated strip, and the completed container coated withthe organic finish.

The chromium plated and electrochemically treated ferrous metalsubstrate produced in accordance with the present invention is notdiscolored. The treated surface is bright, and varies between grey andsilver-grey in appearance. This is of importance as often it is desiredto apply clear organic coatings.

The foregoing detailed description and the following specific exampleare for purposes of illustration only, and are not intended as beinglimiting to the spirit or scope of the appended claims.

EXAMPLE -Coils of 55 pounds double reduced blackplate are cleaned by aprior art procedure to remove grease, dirt and other foreign matter fromthe surface. The treatment includes cathodic and anodic electrolytictreatment in a caustic cleaner (Orthosil), rinsing with water, picklingin dilute (2-3%) aqueous sulfuric acid, scrubbing and rinsing with waterand drying. The clean strip is then passed continuously through a seriesof vessels in a pilot line wherein it is electroplated with metallicchromium, rinsed with water, electrochemically treated, rinsed withwater and dried.

The electrolyte for the chromium plating bath contains 250-275 grams perliter of chromic acid, and sulfate and silicofluoride ion as a catalystin a mole ratio of chromic acid to catalyst of :1 to 100:2. Thetemperature of the electrolyte is maintained at about F., and thecurrent density at about 800-],000 amperes per square foot. The anodesin the chromium plater are lead coated steel on the surfaces facing thestrip, and are coated with polyvinyl chloride on the opposite sidethereof so as to eliminate stray currents. The amount of metallicchromium deposited on the strip is varied on different runs to producecoatings having thicknesses of 0.1, 0.2, 0.3, 0.4 and 0.5 microinch.

The metallic chromium plated strip is withdrawn from the chromiumplater, rinsed with water, and is then passed into an electrochemicaltreating vessel filled with an elec trolyte containing 80 grams perliter of chromic acid. The electrolyte temperature is maintained at 120F., and the strip surface is treated cathodically for 2 seconds at 50amperes per square foot to thereby provide a surface treatment of 100coulombs per square foot. The strip is passed between steel anodescoated with lead on the surface facing the strip. This treatment resultsin about 1-2 milligrams per square foot of total chromium in the filmdeposited on the strip. The chromium is present in the film in the formof a mixture of metallic chromium and chromium oxide in which thechromium has a valence of plus 3. About to /2 of the total chromium inthe film is metallic (Cr).

The treated strip is withdrawn from the electrochemical treating vessel,rinsed in water, dried and coiled. Samples of strip are cut from thevarious runs for the dilferent metallic chromium coating weights andtested for corrosion resistance by the underfilm corrosion test. A peeladhesion test was also conducted.

The peel adhesion test is based on the relative peel strength of anadhesive bonded between two previously lacquered strips of metal. Themetal samples for testing are trimmed to a convenient size (6 x 11inches), lacquered with epoxy lacquer to provide a dry film weight of1.5-2.0 milligrams per square inch, and the lacquered panels are curedin an oven for 10 minutes at 400-405 F. The lacquered panels are shearedinto inch x 5% inch strips. A film of nylon adhesive having a size of /2inch x inch is heat tacked over a hotplate to the ends of lacquered teststrips. Another test strip is placed over the adhesive and the specimenis ready for bonding. The test samples are heat bonded on a hydraulicpress with platen temperatures set at 450 F. and at a gauge pressure of8,000 pounds. The bonding cycle for each specimen is maintained for 5seconds. With each sample, the platens are shimmed to allow a 3 miladhesive layer to remain between the bonded strips. After the bond isformed, the samples are cooled to room temperature and aged overnight.The specimens are tested using an Instron tensile tester operated at acrosshead speed of 1 inch per minute. The sample pieces are gripped onone end and peeled at 180 over /2 inch peel rolls. The results of thistest are reported from the values of high and low peel strength obtainedwith each set of specimens. A minimum and maximum value is reported fromthe average peel strength values for 12 specimens. This is indicated inpounds of peel strength per inch specimen.

Five series of samples having chromium thicknesses of 0.1, 0.2, 0.3, 0.4and 0.5 microinch had minimum average peel strength values of 39.4-48.9pounds, and maximum average peel strength values of 61.4-65.1 pounds.Values above 20 pounds are considered to be satisfactory, and thus theproducts of this example exceeded the minimum specifications by a widemargin. Tests on a prior art sample having a metallic chromium coatingthickness of about 2 microinches gave maximum peel strength values of50-70 pounds, the average of which is 60 pounds, or a little lower thanfor the products of the invention. Therefore, the products of thisexample are fully comparable or better than the prior art products withrespect to peel strength test values.

'In the underfilm corrosion test, a 6 inch x 9 inch panel for eachmetallic chromium coating weight is coated with 3-4 milligrams persquare inch of an epoxy-urea-formaldehyde organic coating. The coatedpanels are cut into one inch strips and scribed with an X through theorganic coating using a sharp tool to expose base steel. The entirescribe areas of the strips are immersed in the test medium, which is alemon-lime carbonated beverage, for 2 weeks at 80 F. The beverage ismaintained under a carbon dioxide atmosphere as described in theAmerican Iron and Steel Institute report entitled Corrosion ResistantTinplate for Carbonated Beverage Containers by Mittleman et al., whichwas presented at the technical meeting on Nov. 18, 1965 and laterprinted.

After two weeks, the samples are removed from the test medium, rinsedwith water, dried and examined under 10, 30 and 60 power magnificationfor indications of underfilm corrosion. No trace of underfilm corrosionwas found on strip samples having metallic chromium coating thicknessesof 0.1, 0.2, 0.3, 0.4 and 0.5 microinch, and thus all samples passed the2 week underfilm corrosion test with a rating of excellent. A 2 weekunderfilm corrosion test on a prior art sample having a chromium coatingthickness of about 2 microinches gave no better results, and thus theproducts of this example are comparable.

What is claimed is:

1. A method of improving the corrosion resistance of ferrous metal stripcomprising the steps of continuously introducing the ferrous metal stripinto a chromium plating bath and continuously electroplating thereon acoating of metallic chromium having a thickness of 0.1-0.5 microinch,the chromium plating bath comprising 100-400 grams per liter of chromicacid and chromium electroplating catalyst selected from the groupconsisting of sulfate ion, fluoride ion, silicofluoride ion and mixturesthereof, the chromium coated ferrous metal strip being subject tocorrosion in the absence of further treatment, continuously withdrawingthe chromium coated ferrous metal strip from the chromium plating bath,thereafter continuously passing the chromium coated ferrous metal stripthrough an aqueous electrolyte for electrochemically treating the same,and continuously electrochemically treating the chromium coated ferrousmetal strip as a cathode at a current density of at least 50 amperes persquare foot in said aqueous electrolyte for the electrochemicaltreatment to deposit a metallic chromium and chromium oxide-containingfilm thereon and increase the corrosion resistance, said aqueouselectrolyte for the electrochemical treatment having a temperature ofabout -150 F. and containing less than grams per liter of chromic acid,and said metallic chromium and chromium oxide-containing film containinga total of 0.6-5 milligrams of chromium per square foot.

2. The method of claim 1 wherein the ferrous metal strip is of containerstock gauge, and an organic coating is applied over theelectrochemically treated ferrous metal strip to providecorrosion-resistant container stock.

3. .The method of claim 2 'wherein the coating of metallic chromium hasa thickness of 0.1-0.3 microninch.

4. The method of claim 3 wherein the metallic chromium coating is porousand metallic chromium is deposited in the pores during the deposition ofthe metallic chromium and chromium oxide-containing film to therebyreduce the porosity, a total of 1-3 milligrams of chromium per squarefoot is present in the chromium oxide and the metallic chromiumcontained in the film, and the organic coating is selected from thegroup consisting of varnishes, lacquers and enamels.

5-. The method of claim 4 wherein the coating of metallic chromium has athickness of about 0.1 microinch.

6. The container stock prepared by the method of claim 2.

7. The method of claim 1 wherein the metallic chromium coating is porousand metallic chromium is deposited in the pores during the deposition ofthe metallic chromium and chromium oxide-containing film to therebyre'duce the porosity, and a total of 1-3 milligrams of chromium persquare foot is present in the chromium oxide and the metallic chromiumcontained in the film.

8. The corrosion resistant ferrous metal strip prepared by the method ofclaim 1.

9. The method of claim 1 wherein the coating of metallic chromium has athickness of 0.1-0.3 microinch.

10. The method of claim 1 wherein the coating of metallic chromium has athickness of about 0.1 microinch.

References Cited UNITED STATES PATENTS 2,746,915 5/1956 Giesker et al.20456 3,118,824 1/1964 Yonezaki et al. 20456 3,245,885 4/1966 Asano etal. 20435 3,296,100 1/1967 Yonezaki et al. 20441 3,113,845 12/1963Uchida et al. 29-183.5

FOREIGN PATENTS 451,904 8/ 1936 Great Britain. 706,377 3/ 1965 Canada.

GERALD L. KAPLAN, Primary Examiner US. Cl. X.R.

